The present invention relates to a novel optical device to which a non-linear phenomenon is applied and, more particularly, to a novel functional optical device using multilayer media having different refractive indices in accordance with the polarizing direction of the light wave.
Many various devices have been proposed as optical devices using the non-linear phenomenon. As a typical device, for instance, a device for causing second-harmonic generation (SHG) has been known. The SHG relates to a method whereby a laser beam having an angular frequency .omega., is irradiated to a non-linear optical crystal and a light wave having the angular frequency 2.omega. is obtained. To efficiently perform the conversion into the second harmonics, it is necessary to select a material having a large non-linear coefficient and to execute the phase matching for making the phase velocity of the fundamental wave coincide with that of the harmonics.
At present, a laser in a visible light range is not put into wide practical use yet because of limitations on material technique. However, a laser in the visible light range can be realized by generating the second harmonics from a laser beam in the infrared range or near the infrared range. This has at present been put into practical use by means of a non-linear material. The high storing density of a optical disk can be realized by using a laser beam in the visible light range with wavelengths shorter than the wavelength of the conventional laser beam in the infrared range.
Hitherto, an optical crystal such as a KH.sub.2 PO.sub.4 (KDP) crystal or the like has been used to generate the second harmonics. On the other hand, a technique to generate the second harmonics by using a semiconductor is desirable from the viewpoint of providing integration with a semiconductor laser and with the manufacturing technique. However, although a compound semiconductor such as GaAs, Inp, or the like has a large non-linear coefficient, there is no means for giving anisotropy to a medium which is optically isotropic. Thus the second harmonics cannot be efficiently generated. It is a technical subject to solve such problem.
An optical device using the non-linear optical effect of a semiconductor material has been described in IEEE, "Journal of Quantum Electronics", Vol. QE-7, No. 11, pages 523 to 529, 1971 (hereinafter, referred to as paper 1). On the other hand, in "Soviet Physics JETP", Vol. 2, No. 3, pages 466 to 475, 1956 (hereinafter, referred to as paper 2), there has been disclosed a fact that a multilayer film obtained by laminating two kinds of materials which are optically isotropic at a period which is sufficiently-shorter than the wavelength of the light wave exhibits optical characteristics which are equivalent to those of a negative uniaxial optical material.
In paper 1, nothing is considered with respect to having means for providing anisotropy to the media having optically isotropy such as compound semiconductors of the III-V group such as GaAs, Inp, and the like. There is a problem in that although such materials have large non-linear optical constants, the non-linear optical effect, such as the second-harmonic generation or the like, cannot be used. On the other hand, in paper 2, a theoretical examination has been made with respect to the case where two kinds of optically isotropic thin films were periodically laminated, so that there has been obtained the fact that such a laminated multilayer exhibits the optical characteristics which are equivalent to those of a negative uniaxial optical material; however, there is nothing in this paper with regard to the development to a new optical device.
As mentioned above, a practical optical device using the non-linear phenomenon of the semiconductor material has not yet been realized.
The present inventors et al. have executed various experiments and examinations in order to artificially give optical anisotropy to a semiconductor material having a large non-linear optical constant, so that they obtained the result that it is possible to realize optical devices having new functions which cannot be obtained with an individual sole thin films under a special condition.
The present invention is made on the basis of the above result and it is an object of the invention to solve the above new subject and to provide an optical device having the novel function.